I've been slowly building and tuning my first quad and have noticed a small anomaly I can't figure out.
For some reason, my GPS' relative altitude is showing a negative value both before takeoff and after landing. The value is not consistent across flights nor is the value the same before takeoff and after landing.
I've attached a few graphs which show relalt on the left axis and HDOP/NSATS on the right.(Cross posted from Ardupilot forum as the audience here is far larger)
The baroalt tracks almost identically with the relalt tracks shown above.
I can appreciate that if I had poor GPS coverage that the 3D position may be out a little (even if I think the HDOP is probably acceptable) but it doesn't explain why the position starts in negative territory.
Can you suggest a cause or method of further investigation?